Store fai logs on failed debian_container_to_vm

Currently, fai's stdout and stderr is dumped to the same logger as containers use.
Unfortunately, @kdienes ran into a situation where a dns error caused fai to be unable to contact its package server. So fai dropped all packages from the list of packages to install.
You couldn't tell this from the stdout/stderr, but only from software.log in FAI's log directory.
Preserve the entire log directory and dump its location to the logger callback.